Power converter and controller using such power converter for electric rolling stock

ABSTRACT

Provided is a power converter having an inverter ( 13 ) wherein capacitors ( 12 ) are connected in parallel on a direct current side, and a power supply circuit configured to supply the inverter with a direct current from a power supply ( 1 ) and a power storage element ( 14 ). A controller using such power converter is also provided for electric rolling stocks. The power supply circuit is provided with a power supply switch (S 1 ) arranged between the power supply and the inverter, a DC-to-DC converter ( 15 A) arranged between the power storage element and the inverter, and a bypass switch (S 2 ) arranged between the power storage element and the inverter.

TECHNICAL FIELD

The present invention relates to a power converter that utilizeselectric power received from a power source and electric power from apower storage unit capable of storing DC electric power so as to supplya load with electric power through an inverter, and to an electricrolling stock controller utilizing the power converter.

BACKGROUND ART

In recent years, a method has been being developed in which a powerstorage element formed of a secondary battery, an electric double layercapacitor, or the like is applied to an electric rolling stockcontroller; it is known that the electric rolling stock controller isconfigured in such away that superfluous regenerative electric powergenerated while a vehicle is braked during a regenerative period isstored and the stored electric power is utilized while the vehicle isaccelerated during a power running period, so that the kinetic energy ofthe vehicle can effectively be utilized (e.g., refer to Patent Document1). Patent Document 1 discloses that an electric rolling stock travelsby means of electric power from a power storage element, withoutreceiving electric power from an overhead line.

Patent Document 1: Japanese Patent Laid-Open Pub. No. 2006-14395

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Recently, development of secondary batteries and electric double-layercapacitors has been being carried out actively, and the amount ofstorable energy has been enlarged; however, the present technologyrequires a considerably large and heavy power storage element in orderto obtain an energy amount large enough to make an electric rollingstock travel. In this regard however, because the mounting space in anelectric rolling stock is limited, it is important to suppress as muchas possible the size and the mass of a power storage element.Accordingly, it is important to effectively utilize the stored energy ofa power storage element; thus, it is indispensable to improve the energyefficiency of an electric rolling stock controller so as to reduce aloss in the controller as much as possible.

The present invention has been implemented in consideration of theforegoing points; with regard to a power converter having a powerstorage unit and an electric rolling stock controller utilizing thepower converter, the objective of the present invention is to provide apower converter in which, when an inverter is driven with electric poweronly from the power storage unit so as to supply a load with AC power,the loss in stored energy of the power storage unit is reduced and thestored energy of the power storage unit is effectively utilized so thatthe energy efficiency is enhanced; it is also the objective of thepresent invention to provide an electric rolling stock controllerutilizing that power converter.

Means for Solving the Problems

A power converter according to the present invention is provided with aninverter that supplies a load with electric power; a capacitor connectedbetween DC terminals of the inverter; a power supply switch providedbetween one terminal of the capacitor and a power source; a powerstorage unit that stores DC power; a DC-to-DC converter having a reactorand at least one pair of switching elements connected in series forcharging the power storage unit with electric power and dischargingelectric power from the power storage unit, the DC-to-DC converter beingconnected in parallel with the capacitor; and a bypass switch thatconnects the power storage unit in parallel with the capacitor, withoutthe intermediary of the switching elements.

Moreover, a power converter according to the present invention isprovided with an inverter that supplies a load with electric power; acapacitor connected between DC terminals of the inverter; a power supplyswitch provided between one terminal of the capacitor and a powersource; a power storage unit that stores DC power; and a DC-to-DCconverter having a reactor and at least one pair of switching elementsconnected in series for charging the power storage unit with electricpower and discharging electric power from the power storage unit, theDC-to-DC converter being connected in parallel with the capacitor, andthe power converter is configured in such a way that on and off statesof the switching elements are fixed in such a way that, in the casewhere the power supply switch is off, the power storage unit isconnected in parallel with the capacitor.

An electric rolling stock controller according to the present inventionis provided with an inverter that drives a motor; a capacitor connectedbetween DC terminals of the inverter; a power supply switch providedbetween one terminal of the capacitor and an overhead line; a powerstorage unit that stores DC power; a DC-to-DC converter having a reactorand at least one pair of switching elements connected in series forcharging the power storage unit with electric power and dischargingelectric power from the power storage unit, the DC-to-DC converter beingconnected in parallel with the capacitor; and a bypass switch thatconnects the power storage unit in parallel with the capacitor, withoutthe intermediary of the switching elements.

Moreover, an electric rolling stock controller according to the presentinvention is provided with an inverter that drives a motor; a capacitorconnected between DC terminals of the inverter; a power supply switchprovided between one terminal of the capacitor and an overhead line; apower storage unit that stores DC power; and a DC-to-DC converter havinga reactor and at least one pair of switching elements connected inseries for charging the power storage unit with electric power anddischarging electric power from the power storage unit, the DC-to-DCconverter being connected in parallel with the capacitor, and theelectric rolling stock controller is configured in such a way that onand off states of the switching elements are fixed in such a way that,in the case where the power supply switch is off, the power storage unitis connected in parallel with the capacitor.

ADVANTAGES OF THE INVENTION

According to a power converter of the present invention and an electricrolling stock controller utilizing the power converter, there can beobtained a power converter in which, in the case where an inverter isdriven only with electric power from a power storage unit so as tosupply a load with electric power, the energy loss in a DC-to-DCconverter is reduced, whereby the energy stored in the power storageunit can effectively be utilized and an electric rolling stockcontroller utilizing the power converter.

Moreover, in a power converter of the present invention and an electricrolling stock controller utilizing the power converter, in the casewhere an inverter is driven only with electric power from a powerstorage unit so as to supply a load with electric power, the powerstorage unit can be connected with the inverter via switching elementsand a reactor, without adding a bypass switch; therefore, the loss in aDC-to-DC converter is reduced, whereby the energy stored in the powerstorage unit can effectively be utilized, and because the reactor canprevent a ripple current from flowing into the power storage unit, it ismade possible to reduce the loss in the power storage unit and toprolong the life thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the configuration including a powerconverter according to Embodiment 1 of the present invention and anelectric rolling stock controller utilizing the power converter;

FIG. 2 is a chart for explaining the transition, according to Embodiment1 of the present invention, from an overhead line/battery combinationmode to a battery operation mode;

FIG. 3 is a chart for explaining the transition, according to Embodiment1 of the present invention, from the battery operation mode to theoverhead line/battery combination mode;

FIG. 4 is a diagram illustrating the configuration including a powerconverter according to Embodiment 2 of the present invention and anelectric rolling stock controller utilizing the power converter;

FIG. 5 is a diagram illustrating the configuration including a powerconverter according to Embodiment 3 of the present invention and anelectric rolling stock controller utilizing the power converter;

FIG. 6 is a diagram illustrating the configuration including a powerconverter according to Embodiment 4 of the present invention and anelectric rolling stock controller utilizing the power converter;

FIG. 7 is a chart for explaining the transition, according to Embodiment4 of the present invention, from an overhead line/battery combinationmode to a battery operation mode;

FIG. 8 is a chart for explaining the transition, according to Embodiment4 of the present invention, from the battery operation mode to theoverhead line/battery combination mode;

FIG. 9 is a diagram illustrating the configuration including a powerconverter according to Embodiment 5 of the present invention and anelectric rolling stock controller utilizing the power converter; and

FIG. 10 is a diagram illustrating the configuration including a powerconverter according to Embodiment 6 of the present invention and anelectric rolling stock controller utilizing the power converter.

DESCRIPTION OF REFERENCE NUMERALS

-   1. OVERHEAD LINE-   2. POWER COLLECTOR-   3. ELECTRIC ROLLING STOCK CONTROLLER-   4. WHEEL-   5. RAIL-   6. MOTOR-   11. REACTOR-   12. CAPACITOR-   13. INVERTER-   14. POWER STORAGE UNIT-   15A&15B. DC-TO-DC CONVERTER-   16.-19. SWITCHING ELEMENT-   20. REACTOR-   21. CAPACITOR-   22. REACTOR-   S1. POWER SUPPLY SWITCH-   S2. BYPASS SWITCH

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1

FIG. 1 is a diagram illustrating the configuration of an electricrolling stock controller according to Embodiment 1 of the presentinvention. In FIG. 1, electric power received from an overhead line,which is a power source, through a power collector 2 is inputted to anelectric rolling stock controller 3. The electric rolling stockcontroller 3 is configured in such a way as to be connected to an ACmotor 6 as a load and to drive the motor 6 so as to make the electricrolling stock travel. A load current from the electric rolling stockcontroller 3 returns to a rail 5 through a wheel 4.

The electric rolling stock controller 3 is configured with a circuitthat supplies an inverter 13, with which a capacitor 12 is connected inparallel, with electric power received from the overhead line 1, via aswitch S1 as a power supply switch and a reactor 11; a circuit thatsupplies the inverter 13 with DC power from a power storage element 14as a power storage unit, via a DC-to-DC converter 15A; and a switch S2as a bypass switch for supplying the inverter 13 with electric powerfrom the power storage element 14, without the intermediary of theDC-to-DC converter 15A. Here, the power storage element 14 is asecondary battery, an electric double-layer capacitor, or the like. Itis preferable that switches having a mechanical contact are utilized asthe switches s1 and S2; however, even though electronic switches, formedof a semiconductor device whose conduction loss is low, are utilized asthe switched S1 and s2, the effect of the present invention is notconsiderably impaired. In Embodiment 1, an example in which a switchhaving a mechanical contact is utilized will be explained.

The DC-to-DC converter 15A is a circuit capable of performingbidirectional power control, i.e., from the power storage element 14 tothe inverter 13 and from the inverter 13 to the power storage element14, under the condition that an inverter input voltage EFC is largerthan a voltage EB across the power storage element 14; the DC-to-DCconverter 15A is a so-called bidirectional step-down DC-to-DC convertercircuit configured with switching elements 16 and 17 and a reactor 20.Because the operation of the DC-to-DC converter circuit is publiclyknown, explanation therefor will be omitted. In addition, an overheadline voltage ESO, the inverter input voltage EFC, the voltage EB acrossthe power storage element 14 can be detected by respective unillustratedvoltage detectors. In this situation, the switch S2 is provided fordirectly performing electric-power transfer between the power storageelement 14 and the inverter 13, without the intermediary of the DC-to-DCconverter 15A; the switch S2 is the subject matter of the presentinvention.

Next, the operation of the electric rolling stock controller, configuredas described above, according to Embodiment 1 will be explained. First,the operation in a mode (referred to as an overhead line/batterycombination mode, hereinafter), in which an electric rolling stocktravels by arbitrarily transferring electric power to and receivingelectric power from the overhead line 1 or the power storage element 14,will be explained.

In this mode, in the case where the electric rolling stock isaccelerated in a power running manner, electric power from the overheadline 1 is supplied to the inverter 13, and electric power from the powerstorage element 14 is adjusted to an optimal value by the DC-to-DCconverter 15A and then supplied to the inverter 13, so that the motor 6is driven with electric power that is the sum of the electric power fromthe overhead line 1 and the electric power from the power storageelement 14. As a specific example, in the case where the amount ofstored energy in the power storage element 14 is slightly insufficient,the DC-to-DC converter 15A suppresses the power storage element 14 fromdischarging electric power, so that more electric power is received fromthe overhead line 1.

In the case where the overhead line voltage ESO is low, or in the casewhere the amount of stored energy in the power storage element 14 isslightly excessive, the DC-to-DC converter 15A operates in such a way asto supply more electric power from the power storage element 14. In sucha manner as described above, while optimally receiving electric powerfrom both the overhead line 1 and the power storage element 14, themotor 6 is driven so as to make the electric rolling stock travel.

In addition, in the case where the electric rolling stock is brakedduring a regenerative period, the motor 6 comes into a regenerativeoperation, and the DC-to-DC converter 15A operates in such a way as toappropriately distribute regenerative electric power from the inverter13 to the overhead line 1 and the power storage element 14. As aspecific example, in the case where the amount of stored energy in thepower storage element 14 is slightly insufficient, or in the case where,because no load of the overhead line 1 exists, electric power is notsufficiently regenerated, the DC-to-DC converter 15A operates in such away that more electric power is regenerated for the power storageelement 14. In contrast, in the case where the amount of stored energyin the power storage element 14 is slightly excessive, the DC-to-DCconverter 15A operates in such a way as to suppress regenerativeelectric power for the power storage element 14 so that almost allregenerative electric power is produced for the overhead line 1. In sucha manner as described above, regenerative electric power from the motor6 is optimally distributed to the overhead line 1 and the power storageelement 14, whereby the regenerative brake on the electric rolling stockworks.

Next, the operation in a mode (referred to as a battery operation mode,hereinafter), in which the electric rolling stock travels only withelectric power from the power storage element 14, will be explained. Theforegoing battery operation mode is established based on, for example, acase where the electric rolling stock travels in the section where theoverhead line 1 is not installed or a case where, even though theoverhead line 1 is installed, the electric rolling stock travels in asection where no electric power is supplied through the overhead line 1.In the battery operation mode, the switch S1 and the DC-to-DC converter15A (the switching elements 16 and 17 are turned off) are turned off,and the switch S2 is turned on. In the case where the electric rollingstock is accelerated in a power running manner, electric power from thepower storage element 14 is supplied directly to the inverter 13 via theswitch S2, without the intermediary of the DC-to-DC converter 15A, sothat the motor 6 is driven.

In such a manner as described above, while receiving electric power fromthe power storage element 14, the motor 6 is driven; thus, it is madepossible to make the electric rolling stock travel even on a track wherethe overhead line 1 is not installed.

In addition, in the case where the electric rolling stock is brakedduring a regenerative period, the motor 6 comes into a regenerativeoperation, and regenerative electric power from the inverter 13 istransferred directly to the power storage element 14 via the switch S2,without the intermediary of the DC-to-DC converter 15A.

As described above, regenerative electric power from the motor 6 istransferred to the power storage element 14, whereby the regenerativebrake on the electric rolling stock works even on a track where theoverhead line 1 is not installed.

Next, the operation in the case in which transition is made from theoverhead line/battery combination mode to the battery operation modewill be explained, and the operation in the case in which transition ismade from the battery operation mode to the overhead line/batterycombination mode will also be explained. FIG. 2 is a chart forexplaining the transition, according to Embodiment 1, from the overheadline/battery combination mode to the battery operation mode. Asrepresented in FIG. 2, in the overhead line/battery combination modeduring the time period between a time instant t0 and a time instant t1,the switch S1 and the DC-to-DC converter 15A are turned on (theswitching elements 16 and 17 are in a PWM operation); therefore, theoverhead line voltage ESO is applied to the capacitor 12, and thevoltage EB across the power storage element 14 is stepped up by theDC-to-DC converter 15A and then applied to the capacitor 12.Accordingly, an input voltage ES at the stage after the switch S1 isequal to the overhead line voltage ESO, and the inverter input voltageEFC is equal to the input voltage ES.

In this situation, the voltage EB across the power storage element 14 islower than the inverter input voltage EFC; the reason for that is thatthe voltage EB across the power storage element 14 is set to be lowerthan the variation lower limit value of the inverter input voltage EFCthat varies in response to the overhead line voltage ESO, inconsideration of the fact that, as explained above, the DC-to-DCconverter 15A is a circuit capable of controlling electric power onlyunder the condition that the inverter input voltage EFC is higher thanthe voltage EB across the power storage element 14. The aboveexplanation will be complemented below. The typical nominal value of theoverhead line voltage ESO is 600 V for streetcars, 750 V for many ofsubway cars, and 1500 V for suburban trains; however, becauseconsiderably varying depending on the distance between the substationand the electric rolling stock and the traveling condition of anelectric rolling stock, the overhead line voltage ESO received by theelectric rolling stock varies within a range from −40% to +20% of thenominal value. In other words, the voltage EB across the power storageelement 14 is set in such a way as to be maintained lower than theinverter input voltage EFC even in the case where the overhead linevoltage ESO (equal to the inverter input voltage EFC) is −40% of thenominal value, i.e., the variation lower limit value.

Next, at the time instant t1, the overhead line/battery combination modeis ended, and then the switch S1 is turned off. In addition, it ispreferable that, in order to prevent the sudden change in the current,the switch S1 is turned off after the current in the switch S1 isreduced to a considerably small value (after being reduced to a valuethe same as or lower than a setting value) by reducing the output of theinverter 13 to a value the same as or lower than a setting value, or bycontrolling the DC-to-DC converter 15A in such a way that almost or allthe electric power of the inverter 13 is imposed on the power storageelement 14. After that, the DC-to-DC converter 15A is controlled in sucha way that the inverter input voltage EFC and the voltage EB across thepower storage element 14 coincide with each other.

When, at a time instant t2, the inverter input voltage EFC coincideswith the voltage EB across the power storage element 14, the DC-to-DCconverter 15A is controlled in such a way that the state in which theinverter input voltage EFC coincides with the voltage EB across thepower storage element 14 is maintained.

After the state in which the difference between the inverter inputvoltage EFC and the voltage EB across the power storage element 14 isthe same as or smaller than a setting value continues for ΔT1 (at a timeinstant t3), it can be determined that the inverter input voltage EFChas sufficiently stabilized; therefore, the switch S2 is turned on, andthe DC-to-Dc converter 15A is turned off.

When the switch S2 is turned on, the current that flows from the powerstorage element 14 to the inverter 13 changes its path from a routethrough the switching element 16 of the DC-to-DC converter 15A to aroute through the switch S2 and without the intermediary of the DC-to-toconverter 15A. The above explanation will be complemented below. Even inthe case, when the inverter 13 is operated in a power running manner,the DC-to-DC converter 15A is turned off, there exists a current pathfrom thepower storage element 14 to the inverter 13 via a diodeincorporated in the switching element 16; however, because the diodeincorporated in the switching element 16 is formed of a semiconductor,the forward voltage drop (several volts) works as a kind of resistor;thus, the diode has a larger resistance than the switch S2 formed of amechanical contact having a minute contact resistance. Accordingly, whenthe switch S2 is turned on, the current that flows from the powerstorage element 14 to the inverter 13 automatically changes its pathfrom a large-resistance route through the switching element 16 of theDC-to-DC converter 15A to a minute-resistance route through the switchS2. In such a manner as described above, transition can smoothly be madefrom the overhead line/battery combination mode to the battery operationmode.

Next, the operation in the case in which transition is made from thebattery operation mode to the overhead line/battery combination modewill be explained. FIG. 3 is a chart for explaining the transition,according to Embodiment 1, from the battery operation mode to theoverhead line/battery combination mode.

As represented in FIG. 3, in the time period between a time instant t4and a time instant t5, the switch S1 and the DC-to-DC converter 15A areturned off, and the switch S2 is turned on; in the foregoing timeperiod, the power storage element 14 directly transfers electric powerto and receives electric power from the inverter 13, via the switch S2.Accordingly, the inverter input voltage EFC is equal to the voltage EBacross the power storage element 14. At the time instant t5, the switchS2 is turned off, and at the same time, the DC-to-DC converter 15A isactivated so that the switching elements 16 and 17 operate in a PWMmanner. In addition, it is preferable that, in order to preventfluctuation in the current from being caused, turning off the switch S2and activation of the DC-to-DC converter 15A are made after the currentof the power storage element 14 is reduced to a value the same as orsmaller than a setting value by suppressing the current of the inverter13.

After the time instant t5, the DC-to-DC converter 15A is operated toperform a step-up function so as to step up the voltage EB across thepower storage element 14, and controlled in such a way that the inverterinput voltage EFC and the overhead line voltage ESO coincide with eachother. At a time instant t6, the inverter input voltage EFC coincideswith the overhead line voltage ESO.

After the state in which the difference between the inverter inputvoltage EFC (equal to the input voltage ES) and the overhead linevoltage ESO is the same as or smaller than a setting value continues forΔT2, it can be determined that the inverter input voltage EFC (equal tothe input voltage ES) has sufficiently stabilized; therefore, at a timeinstant t7, the switch S1 is turned on so as to implement connectionwith the overhead line 1. After the time instant t7, the inverter 13 canbe operated in the overhead line/battery combination mode in which theoverhead line 1 transfers electric power to and receives electric powerfrom the power storage element 14.

As described above, the switch S1 is turned on after the voltage acrossthe terminals of the switch S1 is made sufficiently low by operating theDC-to-DC converter 15A in a step-up manner, thereby making the inverterinput voltage EFC (equal to the input voltage ES) coincide with theoverhead line voltage ESO; therefore, the voltage difference can beprevented from causing a rush current and damaging the contact of theswitch S1.

In such a manner as described above, transition can smoothly be madefrom the overhead line/battery combination mode to the battery operationmode, or from the battery operation mode to the overhead line/batterycombination mode.

Here, the relationship between the loss in the DC-to-DC converter 15Aand the amount of energy stored in the power storage element 14 will bequantitatively explained with reference to an example. In general, theloss in the DC-to-DC converter 15A is approximately 3%; thus, when aDC-to-DC converter having capacity 500 KW, which is a minimallynecessary amount for driving a single electric rolling stock, is takenas an example, the maximal loss is approximately 15 KW, and the averageloss is approximately 5 KW when the travel pattern (accelerating powerrunning, a coasting travel, repetition of regenerative braking) of theelectric rolling stock is taken into account. Meanwhile, the amount ofenergy stored in the power storage element 14 which can be mounted in anelectric rolling stock is decided by the mounting space in the electricrolling stock; the energy amount that has been put to practical use isapproximately 10 KWh (per car). In other words, the loss of 5 KWsuggests that energy stored by fully charging the power storage element14 is completely dissipated in about 2 hours. As described above, it canbe seen that, because the energy that can be stored in the power storageelement 14 is limited, the loss in the DC-to-DC converter 15A is notnegligible.

As described above, in Embodiment 1 of the present invention, the switchS2 is turned on in the battery operation mode so that the power storageelement 14 transfers electric power to and receives electric power fromthe inverter 13 without the intermediary of the DC-to-Dc converter 15A;therefore, no loss occurs in the DC-to-DC converter 15A, whereby theenergy stored in the power storage element 14 can be utilized maximallyefficiently for driving an electric rolling stock.

Embodiment 2

FIG. 4 is a diagram illustrating a configuration example of an electricrolling stock controller according to Embodiment 2 of the presentinvention. In comparison with Embodiment 1 illustrated in FIG. 1, theconfiguration illustrated in FIG. 4 is characterized in that one of theconnection points for the switch S2 is changed from the positive side ofthe power storage element 14 to the connection point between theswitching elements 16 and 17. When the switch S2 is turned on, the powerstorage element 14 is connected in parallel with the capacitor 12,without the intermediary of the switching elements 16 and 17. Otherparts are the same as those in the case of Embodiment 1; thus, bydesignating the same reference numerals, explanations therefor will beomitted.

According to the configuration of Embodiment 2, it is made possible toconnect the power storage element 14 with the inverter 13 via thereactor 20. By connecting the power storage element 14 with the inverter13 via the reactor 20, a ripple current caused by the PWM operation ofthe inverter 13 can be prevented from flowing into the power storageelement 14. Because, when a ripple current flows in the power storageelement 14, the internal heat increases, thereby becoming a lifeshortening factor for the power storage element 14. By employing theconfiguration according to Embodiment 2, the loss in the power storageelement 14 decreases and the life thereof can be prolonged, although theenergy loss in the reactor 20 slightly increases; thus, there exists amerit as a whole.

In addition, the operation of the electric rolling stock controller,configured as described above, according to Embodiment 2 is the same asthat described in Embodiment 1 (FIGS. 2 and 3); therefore, explanationtherefor will be omitted.

As described above, in Embodiment 2 of the present invention, the switchS2 is turned on in the battery operation mode so that the power storageelement 14 transfers electric power to and receives electric power fromthe inverter 13 without the intermediary of the switching element 16;therefore, neither conduction loss nor switching loss occurs in theswitching elements 16 and 17, whereby the energy stored in the powerstorage element 14 can be utilized maximally efficiently for driving anelectric rolling stock. Moreover, because the reactor 20 can prevent aripple current from flowing into the power storage element 14, it ismade possible to reduce the loss in the power storage element 14 so asto prolong the life thereof.

Embodiment 3

FIG. 5 is a diagram illustrating a configuration example of an electricrolling stock controller according to Embodiment 3 of the presentinvention. In comparison with Embodiment 1 illustrated in FIG. 1, theconfiguration illustrated in FIG. 5 is characterized in that the switchS2 is removed and an operation mode is added to the DC-to-DC converter15A. Other parts are the same as those in the case of Embodiment 1;thus, by designating the same reference numerals, explanations thereforwill be omitted.

As illustrated in FIG. 5, Embodiment 3 is characterized in that theswitch S2 is not provided, and the function thereof is replaced by thatof the switching element 16 of the DC-to-DC converter 15A. In otherwords, at the timing, already explained in Embodiment 1 (FIGS. 2 and 3),when the switch S2 is turned on, the switching element 16 is fixed to anon-state (the switching element 17 is fixed to an off-state). By fixingthe switching element 16 to an on-state, the power storage element 14and the inverter 13 can be connected via the switching element 16 andthe reactor 20. By utilizing the foregoing configuration, the loss thatoccurs in the DC-to-DC converter 15A is only the conduction loss in thereactor 20 and the switching element 16, and there occurs none of theswitching loss in the switching element 16, the conduction loss and theswitching loss in the switching element 17, and the iron loss, due to aswitching current, in the reactor 20 that are caused in the case wherethe DC-to-DC converter 15A is ordinarily operated; thus, the system losscan be reduced, and the addition of the switch S2 is not required.

As described above, in Embodiment 3 of the present invention, the powerstorage element 14 and the inverter 13 can be connected in the batteryoperation mode via the switching element 16 and the reactor 20, withoutadding the switch S2; therefore, the loss in the DC-to-DC converter 15Ais reduced, whereby the energy stored in the power storage element 14can be utilized maximally efficiently for driving an electric rollingstock. Moreover, because the reactor 20 can prevent a ripple currentfrom flowing into the power storage element 14, it is made possible toreduce the loss in the power storage element 14 so as to prolong thelife thereof.

Embodiment 4

FIG. 6 is a diagram illustrating a configuration example of an electricrolling stock controller according to Embodiment 4 of the presentinvention. In comparison with Embodiment 1 illustrated in FIG. 1, theconfiguration according to Embodiment 4 illustrated in FIG. 6 ischaracterized in that the DC-to-DC converter 15A is replaced by aDC-to-DC converter 15B. Other parts are the same as those in the case ofEmbodiment 1; thus, by designating the same reference numerals,explanations therefor will be omitted.

In FIG. 6, the DC-to-DC converter 15B is formed of a so-calledbidirectional step-up and step-down DC-to-DC converter circuitconfigured with switching elements 16 to 19 that perform a PWMoperation, reactors 20 and 22, and a capacitor 21; the DC-to-DCconverter 15B is characterized in that electric-power control can beperformed in an arbitrary direction, regardless of the magnituderelationship between the voltage EB across the power storage element 14and the inverter input voltage EFC. Accordingly, the voltage EB acrossthe power storage element 14 can be set regardless of the variationlower limit value of the overhead line voltage ESO; therefore, thevoltage EB across the power storage element 14 can also be set to avalue equal to the nominal value of the overhead line voltage ESO or toa value higher than the nominal value of the overhead line voltage ESO.

Further explanation will be made below. In the configuration accordingto Embodiment 1, because, as explained above, the voltage EB across thepower storage element 14 is required to be set lower than the overheadline voltage ESO, the inverter input voltage EFC becomes lower in thebattery operation mode than in the overhead line/battery combinationmode; therefore the torque generated by the motor 6 is reduced or in thecase where the same electric power is supplied to the inverter 13, thecurrent in the power storage element 14 increases, whereby the loss maybe increased. In contrast, according to the configuration of Embodiment4, it is made possible to ensure the inverter input voltage EFC, in thebattery operation mode, that is the same as or higher than the inverterinput voltage EFC in the overhead line/battery combination mode.Accordingly, in the battery operation mode, the inverter input voltageEFC does not become lower than in the overhead line/battery combinationmode, whereby the torque generated in the motor 6 can sufficiently beensured, and the traveling performance, of the electric rolling stock,which is equivalent to the traveling performance in the overheadline/battery combination mode can be ensured without increasing thecurrent in the power storage element 14. In the explanation below, acase will be explained in which the voltage EB across the power storageelement 14 is set slightly higher than the nominal value of the overheadline voltage ESO.

Next, the operation of the electric rolling stock controller, configuredas described above, according to Embodiment 4 will be explained. Therespective operations in the overhead line/battery combination mode andthe battery operation mode are similar to those explained in Embodiment1; therefore, explanations therefor will be omitted. Therefore, here,the operation in the case in which transition is made from the overheadline/battery combination mode to the battery operation mode and theoperation in the case in which transition is made from the batteryoperation mode to the overhead line/battery combination mode will beexplained.

FIG. 7 is a chart for explaining the transition, according to Embodiment4 of the present invention, from the overhead line/battery combinationmode to the battery operation mode. As represented in FIG. 7, in theoverhead line/battery combination mode during the time period betweenthe time instant t0 and the time instant t1, the switch S1 and theDC-to-DC converter 15B are turned on (the switching elements 16 to 19are in a PWM operation); therefore, the overhead line voltage ESO isapplied to the capacitor 12, and the voltage EB across the power storageelement 14 is stepped down by the DC-to-DC converter 15B and thenapplied to the capacitor 12. Accordingly, an input voltage ES at thestage after the switch is equal to the overhead line voltage ESO, andthe inverter input voltage EFC is equal to the input voltage ES.

Next, at the time instant t1, the overhead line/battery combination modeis ended, and then the switch S1 is turned off. In addition, it ispreferable that, in order to prevent the sudden change in the current,the switch S1 is turned off after the current in the switch S1 isreduced to a considerably small value (after being reduced to a valuethe same as or lower than a setting value) by reducing the output of theinverter 13 to a value the same as or lower than a setting value, or byperforming control in such a way that almost or all the electric powerof the inverter 13 is imposed on the power storage element 14. Afterthat, the DC-to-DC converter 15B is made to perform step-up operation insuch a way that the inverter input voltage EFC and the voltage EB acrossthe power storage element 14 coincide with each other.

When, at a time instant t2, the inverter input voltage EFC coincideswith the voltage EB across the power storage element 14, the DC-to-DCconverter 15B is controlled in such a way that the state in which theinverter input voltage EFC coincides with the voltage EB across thepower storage element 14 is maintained. After the state in which thedifference between the inverter input voltage EFC and the voltage EBacross the power storage element 14 is the same as or smaller than asetting value continues for ΔT1 (at a time instant t3), it can bedetermined that the inverter input voltage EFC has sufficientlystabilized; therefore, the switch S2 is turned on, and the DC-to-Dcconverter 15B is turned off. As a result, the current that flows fromthe power storage element 14 to the inverter 13 changes its path from aroute through the DC-to-DC converter 15B to a route through the switchS2 and without the intermediary of the DC-to-to converter 15B. In such amanner as described above, transition can smoothly be made from theoverhead line/battery combination mode to the battery operation mode.

Next, the operation in the case in which transition is made from thebattery operation mode to the overhead line/battery combination modewill be explained. FIG. 8 is a chart for explaining the transition,according to Embodiment 4, from the battery operation mode to theoverhead line/battery combination mode. As represented in FIG. 8, in thetime period between a time instant t4 and a time instant t5, the switchS1 and the DC-to-DC converter 15B are turned off, and the switch S2 isturned on; in the foregoing time period, the power storage element 14directly transfers electric power to and receives electric power fromthe inverter 13.

Next, at the time instant t5, as preparation for the transition to theoverhead line/battery combination mode, the switch S2 is turned off, andat the same time, the DC-to-DC converter 15B is activated. In addition,it is preferable that, in order to prevent fluctuation in the currentfrom being caused, turning off the switch S2 and activation of theDC-to-DC converter 15B are made after the current of the power storageelement 14 is reduced to a value the same as or smaller than a settingvalue by suppressing the current of the inverter 13.

After the time instant t5, the DC-to-DC converter 15B is made to operatein a step-down manner so as to step down the voltage EB across the powerstorage element 14 and supply the stepped down voltage to the capacitor12, and controlled in such a way that the inverter input voltage EFC andthe overhead line voltage ESO coincide with each other. At a timeinstant t6, the inverter input voltage EFC coincides with the overheadline voltage ESO. After the state in which the difference between theinverter input voltage EFC and the overhead line voltage ESO is the sameas or smaller than a setting value continues for ΔT2, it can bedetermined that the inverter input voltage EFC has sufficientlystabilized; therefore, at a time instant t7, the switch S1 is turned onso as to implement connection with the overhead line 1. After the timeinstant t7, the inverter 13 can be operated in the overhead line/batterycombination mode in which the overhead line 1 transfers electric powerto and receives electric power from the power storage element 14. Asdescribed above, the switch S1 is turned on after the voltage across theterminals of the switch S1 is made sufficiently low by operating theDC-to-DC converter 15B in a step-down manner, thereby making theinverter input voltage EFC coincide with the overhead line voltage ESO;therefore, the voltage difference can be prevented from causing a rushcurrent and damaging the contact of the switch S1.

In such a manner as described above, transition can smoothly be madefrom the overhead line/battery combination mode to the battery operationmode, or from the battery operation mode to the overhead line/batterycombination mode.

Here, as described above, in comparison with the DC-to-DC converter 15Ain Embodiment 1, the DC-to-DC converter 15B is characterized in that itcan be controlled regardless of the magnitude relationship between thevoltage EB across the power storage element 14 and the inverter inputvoltage EFC. The loss in the DC-to-DC converter 15B is larger than thatin the DC-to-DC converter 15A. However, in the configuration accordingto Embodiment 4, the switch S2 is turned on in the battery operationmode so that the power storage element 14 transfers electric power toand receives electric power from the inverter 13 without theintermediary of the DC-to-Dc converter 15B; therefore, no loss occurs inthe DC-to-DC converter 15B, whereby the energy stored in the powerstorage element 14 can be utilized maximally efficiently for driving anelectric rolling stock. Moreover, in the battery operation mode, theinverter input voltage EFC does not become lower, whereby the torquegenerated in the motor 6 can sufficiently be ensured, and the travelingperformance, of the electric rolling stock, which is equivalent to thetraveling performance in the overhead line/battery combination mode canbe ensured without increasing the current in the power storage element14.

Embodiment 5

FIG. 9 is a diagram illustrating a configuration example of an electricrolling stock controller according to Embodiment 5 of the presentinvention. In comparison with Embodiment 4 illustrated in FIG. 6, theconfiguration, illustrated in FIG. 9, according to Embodiment 5 ischaracterized in that one of the connection points for the switch S2 ischanged from the positive side of the power storage element 14 to theconnection point between the switching element 18 and the reactor 22.Other parts are the same as those in the case of Embodiment 4; thus, bydesignating the same reference numerals, explanations therefor will beomitted.

According to the configuration of Embodiment 5, it is made possible toconnect the power storage element 14 and the inverter 13 via the reactor22. By connecting the power storage element 14 with the inverter 13 viathe reactor 22, a ripple current caused by the PWM operation of theinverter 13 can be prevented from flowing into the power storage element14. Because, when a ripple current flows in the power storage element14, the internal heat increases, thereby becoming a life shorteningfactor for the power storage element 14. By employing the configurationaccording to Embodiment 5, the loss in the power storage element 14decreases and the life thereof can be prolonged, although the loss inthe reactor 22 increases; thus, there exists a merit as a whole.

In addition, the operation of the electric rolling stock controller,configured as described above, according to Embodiment 5 is the same asthat described in Embodiment 4; therefore, explanation therefor will beomitted.

As described above, in Embodiment 5 of the present invention, the switchS2 is turned on in the battery operation mode so that the power storageelement 14 transfers electric power to and receives electric power fromthe inverter 13 without the intermediary of the switching elements 16 to19 and the reactor 20; therefore, neither conduction loss nor switchingloss occurs in the switching elements 16 and 19 and no loss occurs inthe reactor 20, whereby the energy stored in the power storage element14 can be utilized maximally efficiently for driving an electric rollingstock. Moreover, because the reactor 22 can prevent a ripple currentfrom flowing into the power storage element 14, it is made possible toreduce the loss in the power storage element 14 so as to prolong thelife thereof.

Embodiment 6

FIG. 10 is a diagram illustrating a configuration example of an electricrolling stock controller according to Embodiment 6 of the presentinvention. In comparison with Embodiment 4 illustrated in FIG. 6, theconfiguration, illustrated in FIG. 10, according to Embodiment 6 ischaracterized in that the switch S2 is removed and an operation mode isadded to the DC-to-DC converter 15B. Other parts are the same as thosein the case of Embodiment 4; thus, by designating the same referencenumerals, explanations therefor will be omitted.

As illustrated in FIG. 10, Embodiment 6 is characterized in that theswitch S2 is not provided, and the function thereof is replaced by theswitching elements 16 and 18. In other words, at the timing, alreadyexplained in Embodiment 4 (FIGS. 7 and 8), when the switch S2 is turnedon, the switching elements 16 and 18 are fixed to an on-state (theswitching elements 17 and 19 are fixed to an off-state). By fixing theswitching elements 16 and 18 to an on-state, the power storage element14 and the inverter 13 can be connected via the switching elements 16and 18 and the reactors 20 and 22.

By utilizing the foregoing configuration, the loss that occurs in theDC-to-DC converter 15B is only the conduction losses in the reactors 20and 22 and the switching elements 16 and 18, and there occurs none ofthe switching loss in the switching elements 16 and 18, the conductionloss and the switching loss in the switching elements 17 and 19, and theiron loss, due to the switching current, in the reactors 20 and 22 thatare caused in the case where the DC-to-DC converter 15B is ordinarilyoperated; thus, the system loss can be reduced, and the addition of theswitch is not required.

As explained above, in Embodiment 6 of the present invention, the powerstorage element 14 and the inverter 13 can be connected in the batteryoperation mode via the switching elements 16 and 18 and the reactors 18and 22, without adding the switch S2; therefore, the loss in theDC-to-DC converter 15B is reduced, whereby the energy stored in thepower storage element 14 can be utilized maximally efficiently fordriving an electric rolling stock. Moreover, because the reactors 20 and22 can prevent a ripple current from flowing into the power storageelement 14, it is made possible to reduce the loss in the power storageelement 14 so as to prolong the life thereof.

The configurations described in the foregoing embodiments are examplesof the aspects of the present invention and can be combined with otherpublicly known technologies; it goes without saying that variousfeatures of the present invention can be configured, by modifying, forexample, partially omitting the foregoing embodiments, without departingfrom the scope and spirit of the present invention.

For example, although not illustrated, the present invention may beapplied to a power converter configured in such a way that AC powersupplied by a power collector is converted into DC power and inputted tothe inverter 13. Additionally, it is also possible to apply the presentinvention to a so-called auxiliary power source apparatus in which loadssuch as a vehicle air conditioner and an illumination apparatus areconnected to the output terminal of the inverter 13 via devices otherthan a motor, e.g., a transformer and a smoothing circuit, and theinverter is operated with a constant voltage and at a constant frequencyso that constant-voltage and constant-frequency power is supplied to theloads.

INDUSTRIAL APPLICABILITY

In the foregoing embodiments, the aspects of the invention have beenexplained by taking, as examples, cases where a power converter isapplied to the field of electric streetcars; however, the applicationfield of the present invention is not limited thereto; the presentinvention can be applied to various related fields such as fields ofelectric automobiles and elevators.

1. A power converter comprising: an inverter that supplies a load withelectric power; a capacitor connected between DC terminals of theinverter; a power supply switch provided between one terminal of thecapacitor and a power source; a power storage unit that stores DC power;a DC-to-DC converter having a reactor and at least one pair of switchingelements connected in series for charging the power storage unit withelectric power and discharging electric power from the power storageunit, the DC-to-DC converter being connected in parallel with thecapacitor; and a bypass switch that connects the power storage unit inparallel with the capacitor, without the intermediary of the switchingelements.
 2. A power converter comprising: an inverter that supplies aload with electric power; a capacitor connected between DC terminals ofthe inverter; a power supply switch provided between one terminal of thecapacitor and a power source; a power storage unit that stores DC power;and a DC-to-DC converter having a reactor and at least one pair ofswitching elements connected in series for charging the power storageunit with electric power and discharging electric power from the powerstorage unit, the DC-to-DC converter being connected in parallel withthe capacitor, wherein on and off states of the switching elements arefixed in such a way that, in the case where the power supply switch isoff, the power storage unit is connected in parallel with the capacitor.3. An electric rolling stock controller comprising: an inverter thatdrives a motor; a capacitor connected between DC terminals of theinverter; a power supply switch provided between one terminal of thecapacitor and an overhead line; a power storage unit that stores DCpower; a DC-to-DC converter having a reactor and at least one pair ofswitching elements connected in series for charging the power storageunit with electric power and discharging electric power from the powerstorage unit, the DC-to-DC converter being connected in parallel withthe capacitor; and a bypass switch that connects the power storage unitin parallel with the capacitor, without the intermediary of theswitching elements.
 4. The electric rolling stock controller accordingto claim 3, wherein the bypass switch connects the power storage unit inparallel with the capacitor via the reactor.
 5. The electric rollingstock controller according to claim 3, wherein, when the power supplyswitch is off, the bypass switch is on.
 6. The electric rolling stockcontroller according to claim 5, wherein, in the case where the powersupply switch and the bypass switch are off, the DC-to-DC converter isactivated, and the difference between the voltage across the capacitorand the voltage across the power storage unit is smaller than apredetermined value, the bypass switch is turned on and the switchingoperation of the DC-to-DC converter is halted.
 7. The electric rollingstock controller according to claim 5, wherein, in the case where thepower supply switch and the bypass switch are off, the DC-to-DCconverter is activated, and the difference between the voltage acrossthe capacitor and the voltage of the overhead line is smaller than apredetermined value, the power supply switch is turned on.
 8. Anelectric rolling stock controller comprising: an inverter that drives amotor; a capacitor connected between DC terminals of the inverter; apower supply switch provided between one terminal of the capacitor andan overhead line; a power storage unit that stores DC power; and aDC-to-DC converter having a reactor and at least one pair of switchingelements connected in series for charging the power storage unit withelectric power and discharging electric power from the power storageunit, the DC-to-DC converter being connected in parallel with thecapacitor, wherein on and off states of the switching elements are fixedin such a way that, in the case where the power supply switch is off,the power storage unit is connected in parallel with the capacitor. 9.The electric rolling stock controller according to claim 8, wherein onand off states of the switching elements are fixed in such a way that,in the case where the power supply switch is off and the differencebetween the voltage across the capacitor and the voltage of the voltageacross the power storage unit is smaller than a predetermined value, thepower storage unit is connected in parallel with the capacitor.
 10. Theelectric rolling stock controller according to claim 8, wherein, in thecase where the DC-to-DC converter is activated and the differencebetween the voltage across the capacitor and the voltage of the overheadline is smaller than a predetermined value, the power supply switch isturned on.